Coming from Rochester, the difficult and expensive bit is to get yourself from ROC to an airport nearby to Irun. I recommend you use one of the major search engines to locate flights from ROC to either BIQ (Biarritz, France), or BIO (Bilbao, Spain). You can take a bus from either airport to get to Irun.
One route I can recommend is flying to Brussels (BRU) on UA (ROC > EWR or JFK > BRU) then transfer to Brussels Airlines (SN) a codeshare for UA, from Brussels (BRU) to Bilbao (BIO). Take a bus from Bilbao to Irun.
Another route that works well is any airline from ROC to CDG (Paris, de Gaulle), then transfer to the TGV high-speed train at the airport to ride to Bayonne, via Bordeaux. From Bayonne, you can get a bus to Irun.
For reference, Irun is just on the Spanish - French border, south of Bayonne and Biarritz.
If it is much cheaper for you to fly to Madrid (MAD) you will approach Irun from the south. From Madrid (MAD) Iberia has the most flights on Iberia Express. You can also use them to return to MAD direct from Santiago.
So a likely routing coming out of MAD would be: (ROC > EWR or JFK > MAD > EAS). EAS is San Sebastian, Spain. It is very close to Irun, has bus service, and is a very nice, large Spanish seaside town. IMHO, it is worth a couple of days if you can spare it.
Flying what is called an "open jaw" return in the airline industry, you would reverse the journey, except starting from Santiago de Compostela (SCQ): (SCQ > MAD > JFK or EWR > ROC). Open jaw returns usually cost no more than a conventional round-trip in most cases.
Also and FYI, Iberia is part of the One World alliance with American Airlines. So, if it matters to you, you can go all the way via MAD on AA and codeshares.
If you desire to use UA and codeshares, then the routing from the north, via (ROC > EWR or JFK > BRU > BIO) is the most direct way, IMHO. You would have to go "off-brand" to get from SCQ to MAD, but you could fly an "open jaw" return route from Madrid (MAD), or take an ALSA bus from SCQ to MAD, then get to the airport. The return routing (UA) would be: (SCQ (bus or air) > MAD > EWR > ROC).
I hope this helps.